Interaction between crustal-derived felsic and mantle-derived mafic magmas in the Oberkirch Pluton (European Variscides, Schwarzwald, Germany)

Abstract

The composite Oberkirch pluton consists of three compositionally different units of peraluminous biotite granite. The northern unit is relatively mafic (SiO2∼64%) and lacks cordierite. The more felsic central and southern units (SiO2=67.8 to 70.4%) can only be distinguished from each other by the occurrence of cordierite in the former. Mafic microgranular enclaves of variable composition, texture and size occur in each of these units and are concentrated in their central domains. Most abundant are large (dm to m) hornblende-bearing enclaves with dioritic to tonalitic compositions (SiO2=50.8 to 56.3 wt%; Mg#=63 to 41) and fine grained doleritic textures that suggest chilling against the host granite magma. Some of these enclaves are mantled by hybrid zones. Less common are microtonalitic enclaves containing biotite as the only primary mafic phase (SiO2=53.7 to 64.4%) and small hybrid tonalitic to granodioritic enclaves and schlieren. Synplutonic dioritic dikes (up to 6 m thick) with hybrid transition zones to the host granite occur in the southern unit of the pluton. In chemical variation diagrams, samples from unmodified hornblende-bearing mafic enclaves and dikes form continuous trends that are compatible with an origin by fractionation of olivine, clinopyroxene, hornblende and plagioclase. Chemical and initial isotopic signatures (e.g. high Mg#, low Na2O, ɛNd=−1.2 to −5.1, 87Sr/86Sr=0.7055 to 0.7080, δ18O=8.0 to 8.8‰) exclude an origin by partial melting from a mafic meta-igneous source but favour derivation from a heterogeneous enriched lithospheric mantle. Samples from the granitic host rocks do not follow the chemical variation trends defined by the diorites but display large scatter. In addition, their initial isotopic characteristics (ɛNd=−4.5 to −6.8, 87Sr/86Sr=0.7071 to 0.7115, δ18O=9.9 to 11.9‰) show little overlap with those of the diorites. Most probably, the granitic magmas were derived from metapelitic sources characterized by variable amounts of garnet and plagioclase. This is suggested by relatively high molar ratios of Al2O3/(MgO+FeOtot) and K2O/Na2O, in combination with low ratios of CaO/(MgO+FeOtot), variable values of Sr/Nd, Eu/Eu*[=Eucn/(Smcn × Gdcn)0.5] and (Tb/Yb)cn (cn=chondrite-normalized) as well as variable abundances of Sc and Y. Whole-rock initial isotopic signatures of mafic microtonalitic enclaves (ɛNd=−4.6 to −5.2; 87Sr/86Sr=0.7060 to 0.7073; δ18O ∼8.1‰) are similar to those of the low ɛNd diorites. Plagioclase concentrates from a granite sample and a mafic microtonalitic enclave are characterized by initial 87Sr/86Sr ratios that are significantly higher than those of their bulk rock systems suggesting incorporation of high 87Sr/86Sr crustal material into the magmas. Field relationships and petrographic evidence suggest that the Oberkirch pluton originated by at least three pulses of granitic magma containing mafic magma globules. In-situ hybridization between the different magmas was limited. Late injection of dioritic magma into the almost solidified granitic southern unit resulted in the formation of more or less continuous synplutonic dikes surrounded by relatively thin hybrid zones.